50/60mm Prime or 28-105mm - Advice Please

I have a Nikon D3 coupled with a Tamron 28-75 2.8 and shoot mostly small groups of people and animals - sometimes in low light in large buildings. I quite like the Tamron but feel it is a little soft mainly at the corners and just isn't quite sharp enough. you can see some of mine at www.supashots.com

I am considering adding a lens or two but can't afford anything like the 24-70mmm 2.8.

I have read all the various lens tests but would like some opinions who live in the Real world and take proper photos

Could anyone please give me their preference for any of the following and reasons why. My main requirement is sharp crisp photos with true colors.